Exhaust FAQ

The primary purpose of an aftermarket exhaust is to remove or replace the stock catalytic converter(s) with a better flowing unit. It also increases the exhaust diameter for better flow. The term “aftermarket exhaust” can be broken down into the three main types:

1. Turbo back exhaust (TBE) covers items 1-4 as described below.
2. Cat back exhaust (CBE) covers items 3&4 as described below.
3. Axle back exhaust (ABE) covers item 4 as described below.

The stock WRX Subaru exhaust consists of:

1. Downpipe (with cat) | 2. Midpipe (with cat) | 3. Rear Exhaust Pipe | 4. Axle Back Muffler

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Keep in mind that the above information is catered to the 2002-2005 WRX and is likewise applicable to all years of the STi, though there are slight variations. For example, the STi (all years) has a catless uppipe and the 2006+ WRX has a one piece downpipe/midpipe.

HP gain is dependent on the type of exhaust chosen. TBE gains are 20-30HP. CBE gains are 5-15HP. ABE gains are 5-10HP. These figures are highly debated as different manufacturers use different dynos with different cars with different levels of mods. It also varies because exhausts use different amounts of catalytic converters or are catless.

What about overall fitment between the WRX and STi or different model years? All OEM or aftermarket exhausts designed for 02-07 WRX/STi will fit either the 02+ WRX sedan/wagon or 04+ STi. Occasionally, you will have a slight fitment issue with some models (moreso if your car has the optional rear differential cover) that usually get be fixed via hanger adjustment, longer exhaust hangers etc. 06+ models have fitment issues with some exhausts (mainly the muffler portion) due to the new rear diffuser.

I have an 06+ WRX, is the downpipe the same? No. The 2006+ WRX has a unique exhaust in that it’s downpipe is both the downpipe and midpipe sections of the “older” exhaust. This means you must use a “long” downpipe to bolt up to the rest of your exhaust system. A full TBE will fit fine, but when replacing just the downpipe, you must use a long downpipe. Check with your Vendor for other possible fitment issues prior to ordering.

Can I use an OEM STi exhaust on my WRX? Yes, but do not use it thinking you now have an upgraded exhaust. While the piping is slightly larger in diameter, the catalytic converter is larger and more restrictive so any gains are negated. About the only reason to swap out components is to go to the STi axleback muffler which is slightly louder than the OEM WRX unit.

How do I stop my car from sounding like a “Honda Killer Bee Exhaust”? Subaru boxer configured engines, even equipped with a “Honda Killer Bee Exhaust” will NOT sound like a buzzy, annoying exhaust due to the motor’s natural sound. The only way to become buzzy is to switch to an equal length header. The OEM exhaust manifold and aftermarket unequal length manifolds (headers) retain the exhaust pulses at different intervals which create the unique boxer sound. This is why your stock exhaust sounds similar to a VW Bug or Porsche engine.

Which manufacturer is best? This topic is highly debated. There have been no reported consistent “bad” exhausts on the market. Obviously, there may have been bad exhausts sold, but not enough to report as “bad” overall.

What exhaust metal material is best? Exhausts are made from mild steel, stainless steel (304 & 321), and titanium. There is no irrefutable evidence that one material is better than the other. Obviously, corrosion levels are higher with mild steel (coated or otherwise). Article on exhaust materials.

Which exhaust diameter is best? Exhausts have 3 diameters:

1. 2.5″: Good for 300-350 HP
2. 3.0″: Good for 500-600 HP
3. 3.5″: Good for 600HP +

There is no irrefutable evidence that one diameter is better than the other. Purchasing a larger exhaust than needed for your level of modification has no disadvantages other than cost and possible less ground clearance issues.

What other differences are there with exhausts? The other differences are as follows:
1. High flow cat(s) or catless.
2. Exhaust pipe length. Some manufacturers have proprietary exhaust component lengths. This means you have to use all of their exhaust piping or some of it with a custom piece. As well, JDM specification exhausts are different length and require all JDM specification piping or some of it with a custom piece. Additionally, some manufacturer’s components may take the place of two OEM components. This generally occurs with the downpipe section of a TBE where the “downpipe” is actually a downpipe+midpipe and these are commonly referred to as “long downpipes”.
3. Flanges/mating surfaces. Exhaust components are either equipped with a flat flange or have a donut gasket designed to flex during movement. Flat goes to flat and donut goes to donut, ensure you double check prior to purchase which you have and what your new exhaust has. Some companies do make adapters of various kinds to mate say diameter A to diameter B piping and/or flat flange to donut gasket.

Which exhaust provides the least back pressure? No one is really sure, but theory suggests that larger piping is better than smaller, baffleless mufflers are better than baffled mufflers, and no cats/freer flowing cats are better than OEM/poorer flowing cats. This thread provides limited testing that bears this theory out.

Which exhaust has the best sound? This subject is up to the individual. It is highly subjective and can lead to trouble with local noise abatement ordinances. That great sounding exhaust may be the cause of some expensive and/or time consuming noise abatement tickets. Arguably, the three most famous/popular “best” sounding exhausts are the ScoobySport, Stromung, and Prodrive. A smart move is to find someone in your local area with the exhaust component(s) you are interested in to hear it in person. Seek out locals via the NASIOC Chapters or NASIOC Regional Areas.

Which exhaust is the quietest? This question can be answered by listing hard data. By using hard data, you remove human judgement and don’t get stuck with a recommend “quiet exhaust” from someone that in your opinion is now a loud and expensive nuisance. Keep in mind that these quiet exhausts only protect you from the chance encounter in areas with strict noise abatement laws. At wide open throttle (WOT) any aftermarket exhaust is considerably louder than OEM and even the quiet ones will not protect you from folly.

Full dB testing data can be found on this link.

Which exhaust is the loudest?

Full dB testing data can be found on this link.

How do I quiet down an already loud exhaust?
1. Install an aftermarket silencer. This is an insert that is fitted into your tailpipe.
2. Install a resonated exhaust tip.
3. Have an inline resonator welded into your exhaust.
4. Install an auger style muffler.
5. Install this device or this device.
6. View this thread for sound deading tips and a new found source of noise.

One special note about silencers. While they do quiet down your exhaust, they generally work by restricting your exhaust flow down to a 1 1/2″ hole. Strongly consider what this does to your backpressure and the effects of backpressure on a turbocharged engine.

Which exhaust has the best gains? There has not been a 100% conclusive test of every aftermarket exhaust to prove or disprove best gain level. Suffice it to say though that theoritically, the exhaust with the largest pipe diameter along with a 100% baffless muffler would be better than a smaller diameter and/or baffled muffler.

Are there any downsides to exhausts? There have not been significant amounts of problems with exhausts. For STi and some 2.5L WRX owners, there have been reports of overboosting issues with some TBEs. For more information read this post and the follow on posts it contains.

Where do I buy an exhaust? Every Subaru/Import performance store sells exhausts. For purchasing, support your local economy or the NASIOC Vendors.

What should I decide upon before purchasing to narrow down my decision?
a. Sound level.
b. Exhaust tip(s) diameter and shape.
c. Catalytic converter issues with regard to your particular state’s emissions laws.
d. Outside appearance, straight back exhaust or angled exhaust?

What questions should I ask of the retailer or manufacturer before purchasing?
1. Will this unit mate up to the rest of my exhaust? Some exhausts will fit the stock exhaust, or must be fitted to a larger exhaust.
2. Will this unit throw a CEL? Some do, some don’t, so find out beforehand.
3. Does it come with new gaskets? It is always a good idea to replace exhaust gaskets.
4. Does it come with a CEL fix if needed and is it the right one? There are two unique electrical fixes, one for the 02/03 and one for the 04. The 05+ does not currently have an electrical fix.
5. Will this exhaust clear the rear differential protector, if applicable?
6. Does this exhaust drone? Drone is a term to describe an annoying monotonous buzzing sound generally heard during long highway trips. This sound can be more apparent with wagons, due to the lack of a trunk to deaden the sound.
7. For 06 users: Will this exhaust clear my rear diffuser?

If after installation, my exhaust drones, how do I fix it? Many users report great success by using audio sound deadening material in the trunk, back seat area, and/or underneath your vehicle. View this thread for sound deading tips and a new found source of noise in the rear.

What is some good background information about exhaust pipe fitment?
Generally speaking, there are two types of catbacks:
Type 1: It has a sliding flange designed to bolt up to the stock downpipe.
Type 2: It has a flat flange designed to bolt up to 3″ downpipes.

Generally speaking, there are two types of downpipes:
Type 1: It is designed to bolt up to the stock catback and other type 1 catbacks.
Type 2: It is designed to bolt up to only type 2 downpipes.

Type 1 catbacks do not bolt up well to type 2 downpipes, and type 2 catbacks do not bolt up well to type 1 downpipes.
Type 1 catbacks and downpipes taper to 2.5″ where they meet.
Type 2 catbacks and downpipes are full 3″.
Type 2 downpipes are usually less expensive than type 1 downpipes.
Type 1 and 2 catbacks are relatively similar in price.

All 2.5″ catbacks are type 1.
Some 3″ catbacks are type 1, and some are type 2.

How hard is it to install an exhaust? Allow around one hour for install time. Professional installation, depending on your area, is around $75. This is one vehicle modification that is very simple and can be successfully accomplished by even the greenest shade tree mechanic.

How do I install an exhaust? Refer to the exhaust manufacturer’s instructions. For exhausts without instructions, below is a link to one of the better known exhaust installation instructions:
TurboXS’s Instructions
Perrin’s Instructions

Do I need aftermarket exhaust hangers? This can depend on many factors and the only clear indication is during the post installation driving test. Some exhausts work perfectly fine with the OEM hangers and some can benefit from the added stiffness or extended length of the aftermarket hangers provide. They are fairly inexpensive and can be good insurance for a perfect, rattle free exhaust install.

What about ceramic or other heat wrap coating? Though there are pros and cons to heat retention treatments with exhaust components, the general consensus with headers is that heat retention benefits outweigh the liabilities.
Pros:
Increased heat retention, which is beneficial for exhaust flow.
Reduces underhood temps, which decreases IC heatsoak.
Cons:
Additional expense.
Additional lead time, in the case of coatings.
Increased heat retention, which in some cases, can accelerate metal fatigue, especially at weld joints.
No proven HP increase.
If you plan on using a coating, ensure you are aware of what impact this will have on your header warranty if applicable. Coating or wrapping your header can void exhaust component warranties. If using a ceramic-type coating, ensure it’s working heat range is above the known EGT. EGTs can be as high as 1400-1600 degrees in the extremes. Some normal coatings are not rated to this temperature level.

What types of ceramic or other heat wrap coating are available? The two most popular manufacturers of heat wrap are Thermo Tec and DEi. The two most popular coating manufacturers are Jet Hot and Swain Tech Coatings. Wrapping is a DIY project with a shorter life span than the professional only coating with a longer life span.

Excellent coating thread
.pdf file on coatings
Car Craft magazine’s testing of Jet Hot

What is the CEL fix? It stops the rear CEL code P0420 Catalyst System Efficiency Below Threshold from appearing, this code being caused by the lack of a catalytic converter in the exhaust. This is usually fixed with a Pony Express MIL Eliminator or similar fix. Ensure you obtain the correct type for your model year as there are two types: 02/03 and 04. The 05+ does not currently have an electrical fix. Another fix is the mechanical fix, which works on all models. ***NOTE*** There are two CEL fixes for the WRX, the other one is the 2.2 KOhm 1/2 watt resistor. It is used to combat the CEL code P0546 = Exhaust Gas Temperature Sensor Circuit Malfunction (High Input) in the uppipe. This notice, though not important to this thread, acknowledges there are two “CEL fixes” and defines them individually to avoid possible confusion.

Which is the best CEL fix, mechanical or electrical? Both work fine, though personnel living in states with OBDII testing prefer the mechanical fix. The reason for this is during state mandated OBDII testing, the electrical fix can show up as a malfunctioning sensor. For some states, this one malfunctioning sensor is fine, for others, you will have to fix it and return. In the end, the mechanical fix is cheaper and easier for most users.

Should I reset my ECU after this mod? It is never a bad idea to perform an ECU reset after a mod. The traditional route is to disconnect the negative battery terminal, press the brake pedal for a few seconds to bleed the system of charge, and reattach. Some use the more advanced Vishnu Reset.

What about emissions problems like emissions tests and legality? Using a catless exhaust is a violation of section 203 of the Clean Air Act. Each state has statutes that cover this subject as well. As far as passing state emissions tests, as long as you still have one cat in your system, you should be fine unless you run into a sharp-eyed and knowledgeable inspector who knows there should be two in the main exhaust. The exhaust should be properly warmed prior to any testing to ensure it is operating efficiently. Another smart move is to find someone in your local area with the same set up to compare notes. Seek out locals via the NASIOC Chapters or NASIOC Regional Areas. (A nice article on cat testing can be found here.)

What are some other good articles on exhausts?
Vishnu Design 101
Exhaust Design, Turbo and Naturally Aspirated

Editors Note

This post was created because I wasn’t able to find a good exhaust FAQ. I came up with the text based on LOTS of searching here. It was also created to be as brand neutral as possible so that it serves as a stepping stone for further research. Certain brand names were mentioned in this post though ONLY because they are the specific answers to specific questions. Upon reading this you should have an idea of what type of exhaust best suits your needs. The manufacturer is up to you.

If you find an error in this FAQ, please PM me with factual details and I will update this post. Responses such as, “I have XXX’s exhaust and it’s great!” or “XXX’s exhaust leaked after 1 month” are not appreciated here, that is what the Car Parts Review Forum is for.

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